Polylactides (or polylactic acids) are a type of resin including a repeating unit of the following General Formula. Unlike conventional petroleum-based resins, the polylactide resins, which are based on biomass, can utilize renewable resources, and their preparation generates less greenhouse gas, CO2, than the preparation of other conventional resins. Also, not only do they have eco-friendly attributes such as biodegradability by water and microorganisms when being buried, but they also possess suitable mechanical strength comparable to the conventional petroleum-based resins.

The polylactide resins have been used mainly for disposable packages/containers, coatings, foams, films/sheets, and fibers. Recently, more efforts have been made to enhance the properties of the polylactide resins by mixing them with conventional resins such as ABS, polycarbonate, or polypropylene, and then utilizing them in a semi-permanent use such as for exterior materials of cell phones or interior materials of vehicles. However, the polylactide resins tend to biodegrade in and of themselves due to factors such as the catalyst used in their preparation, moisture in the air, and the like, and up to now such drawbacks of their own properties have limited their application.
Meanwhile, previously known processes for preparing the polylactide resins involve either directly subjecting lactic acid to condensation polymerization or carrying out ring opening polymerization with lactide monomers in the presence of an organometallic catalyst. In this regard, the direct condensation polymerization can produce the polymer at a low cost but it is difficult to obtain the polymers having a high molecular weight in terms of a weight average molecular weight of 100,000 or more, making it difficult to sufficiently ensure the physical and mechanical properties of the polylactide resins. In addition, the ring opening polymerization of the lactide monomers entails a higher cost than the condensation polymerization since the lactide monomers should be prepared from lactic acid, but it can produce a polymer having a relatively high molecular weight and is advantageous in controlling the polymerization, and thus it is commercially used.
Representative examples of the catalyst as used in such ring opening polymerization include a Sn-containing catalysts such as Sn(Oct)2 (Oct=2-ethyl hexanoate). However, this catalyst not only promotes the ring opening polymerization, but also tends to accelerate the depolymerization at a conversion rate exceeding a certain level (see U.S. Pat. No. 5,142,023; Leenslag et al. Makromol. Chem. 1987, 188, 1809-1814; Witzke et al. Macromolecules 1997, 30, 7075-7085). Accordingly, the polylactide resin prepared from the ring opening polymerization tends to have a decreased molecular weight, a broadened molecular weight distribution, and an increased amount of remaining monomers, all of which can have an undesirable effect on the polymer properties.
In other words, the ring opening polymerization of the lactide as described above is a reaction involving a thermodynamic equilibrium between the monomers and the polymer, in which the conversion rate to the polylactide resin increases at the beginning as the polymerization time passes, but the reaction reaches some degree of equilibrium when the conversion rate no longer increases. This also means that the resulting polylactide resin after the polymerization essentially contains a certain amount of the monomer therein. Generally, it has been known that as the reaction temperature becomes higher, the amount of the monomer at the equilibrium state increases, while the reverse holds true as the reaction temperature is reduced. Not only do the monomers remaining in the polylactide resin after the polymerization have detrimental effects on the mechanical properties of the resin, but they also tend to be hydrated, causing corrosion at the time of processing, and can accelerate the decomposition via the depolymerization of the resin. Accordingly, it is very important to control the amount of the monomer remaining after the polymerization.
Due to the foregoing drawbacks, even though the ring opening polymerization previously known in the art is applied, it is difficult to obtain a polylactide resin with a sufficiently high molecular weight and excellent mechanical properties at a high conversion rate because of the depolymerization. Moreover, in their use, the polylactide resins suffered the decomposition caused by the monomers and the catalyst remaining therein, which in turn brought about serious problems in their properties, such as hydrolysis resistance, heat resistance, and the like. Such problems have hindered efforts to apply the polylactide resins for a semi-permanent use, such as for exterior materials of the cell phones and interior materials of vehicles.
Meanwhile, attempts have been made to suppress the depolymerization or the decomposition of the polylactide resin and to obtain polylactide resins having a higher molecular weight and excellent mechanical properties at a high conversion rate.
First, there was an attempt to carry out ring opening polymerization using a Sn-containing catalyst, in which an amine-based proton trapping agent was added in order to prevent the depolymerization. However, even with this method, which could prevent the acid from lowering the catalytic activity or causing a hydrolysis of the resin to some extent, it was found that the depolymerization by the catalyst or the like still proceeded and it was difficult to obtain polylactide resins having a high molecular weight and excellent mechanical properties (Majerska et al. Macromol Rapid Commun 2000, 21, 1327-1332; Kowalski et al. Macromolecules 2005, 38, 8170-8176).
In addition, the use of a catalyst containing Zn instead of Sn has been considered, but this method has also a drawback of low polymerization activity (Nijenhuis et al. Macromolecules 1992, 25, 6419-6424).
On the other hand, some recent reports revealed that the polymerization activity and the molecular weight are increased when the lactides are polymerized by using a Sn(Oct)2P(Ph)3 compound coordinated by a phosphine compound. Expectedly, this was due to the fact that electrons in the Sn-containing catalyst were localized by the phosphine and thereafter the coordination of the lactide monomers was induced faster (see U.S. Pat. No. 6,166,169; Degee et al Journal Polymer Science Part A; Polymer chemistry 1999, 37, 2413-2420; Degee et al Macromol. Symp. 1999, 144, 289-302). Also, U.S. Pat. No. 5,338,822 discloses a method of preventing the depolymerization, in which the resin melt obtained from the lactide polymerization was subjected to a post-treatment by adding a phosphite-based antioxidant thereto.
Meanwhile, there is a report that hydrolysis resistance of the polylactide resin is improved by kneading the polylactide resin with a carbodiimide compound known to be used as an acid scavenger in polyester processing (Japanese Patent Publication No. 2008-248162).
However, even though hydrolysis resistance can be improved by maintaining the acidity of polylactide resin at a low level by those methods, it is difficult to sufficiently prevent generation of monomer residues by the depolymerization or the resin decomposition thereby. There is another problem that the resin turns yellow or brown after kneading due to the use of chromophore-containing carbodiimide.